The conversion of light hydrocarbons to aromatics is best viewed as a hydrogen disproportionation reaction. For each aromatic molecule formed, a significant number of hydrogen equivalents are generated. The thermodynamics of the disproportionation reaction dictate that the reaction should proceed such that more mass is converted to fuel gases than to liquid aromatic products. Therefore, catalysts which promote the formation of molecular hydrogen, which in turn promotes liquid aromatic production are desired. The production of liquid aromatic products represents the production of premium products from a fuel or chemical standpoint.
The conversion of light hydrocarbons to aromatic hydrocarbons over modified ZSM-5 systems, which are acidic molecular sieves, is known. Catalysis over molecular sieves involves the diffusion of reactants into the microporous channel system of the molecular sieve and the diffusion of the products out of the same system. Modified ZSM-5 zeolite catalysts have been used to catalyze both paraffin dehydrogenation and olefin dehydrocyclooligomerization. These catalysts provide shape selectivity such that large fractions of C.sub.10 + products are avoided. Various modifications and pretreatments of the zeolite catalyst have resulted in improvements in light hydrocarbon conversion and aromatic selectivity, though often one has been achieved to the slight detriment of the other.
Ethane conversion processes disclosed in existing art are very similar to one another in nature: a ZSM-5 zeolite in the acid form is modified with zinc and/or gallium to act as a catalyst for the formation of aromatics from ethane at temperatures near 600.degree. C. Most of the examples of ethane conversion over ZSM-5 catalysts in the patent literature disclose a ZSM-5 catalyst synthesized under agitated conditions to form the intermediate gel phase. This produces submicron crystalline size in the resultant molecular sieve, which tends to cause severe coking of the catalyst.
In early 1986, the art reported maximum per pass conversions (ppc) of light hydrocarbons to aromatics of about 20%. Patents to Mobil, specifically U.S. Pat. Nos. 4,120,910 and 4,350,835, disclose dramatic changes in per pass conversion depending on run time, i.e. 21% ppc at 10 minutes over a CuZnZSM-5 and 11.2% ppc at 40 minutes over the same catalyst. A later patent, to British Petroleum, EP 0 171 981, reported conversion of ethane to aromatics over a simple GaZSM-5 catalyst at high temperatures and relatively low space velocity.
A more recent patent is U.S. Pat. No. 4,613,716, to British Petroleum, disclosing and claiming a GaHZSM-5 catalyst treated with either a Group VIIB or a Group VIII metal promoter for ethane conversion. This art reports slightly improved per pass conversion of ethane to aromatics, in the 21% to 24% range.
It is an object of the present invention to develop a process for producing an ethane conversion catalyst which effectively and efficiently converts ethane to liquid aromatic hydrocarbons.
It is another object of the present invention to develop a catalyst displaying high conversion rates and maintaining good selectivity to aromatics.